Phase-sensitive ground fault protective systems

ABSTRACT

To prevent possible electrocutions and to minimize the risk of fires caused by insulation faults in alternating current power distribution systems, a protective system responsive to ground fault currents in phase with the line potentials interrupts the electrical power. Ground fault currents are detected by a sensor responsive to differential or unbalanced currents in the supply conductors of the distribution system. The unbalanced currents are compared with the phase of the line potentials by a demodulator to detect only in-phase components and produce an output signal representing a true fault, the magnitude of which depends on the degree of current imbalance. Output signals exceeding a threshold value trip a circuit breaker to remove power from the load conductors. The combination is sensitive to ground faults of either high or low resistances, and is substantially insensitive to spurious signals which do not represent true faults.

United States Patent -1 1 1 Gross [54] PHASE-SENSITIVE GROUND FAULTPROTECTIVE SYSTEMS [76] Inventor: Thomas A. 0. Gross, Concord Road RFD,Lincoln, Mass.

[22] Filed: May 1, 1972 [21] Appl. No.: 249,156

UNITED STATES PATENTS Gross ..3l7/l8 D 3,633,070 1/1972 Vassos et al......3l7/18 D 3,657,604 4/1972 Willard ..3l7/l8 D Primary ExaminerJames D.Trammell Attorney-William D. Roberson 1 1 Mar. 27, 1973 57 ABSTRACT Toprevent possible electrocutions and to minimize the risk of fires causedby insulation faults in alternating current power distribution systems,a protective system responsive to ground fault currents in phase withthe line potentials interrupts the electrical power. Ground faultcurrents are detected by a sensor responsive to differential orunbalanced currents in the supply conductors of the distribution system.The unbalanced currents are compared with the phase of the linepotentials by a demodulator to detect only inphase components andproduce an output signal representing a true fault, the magnitude ofwhich depends on the degree of current imbalance. Output signalsexceeding a threshold value trip a circuit breaker to remove power fromthe load conductors. The combination is sensitive to ground faults ofeither high or low resistances, and is substantially insensitive tospurious signals which do not represent true faults.

12 Claims, 3 Drawing Figures F 3 T l lo i 51 3 M l 1,11% l i l I I l l I47 5 l lQQQMMf/ 1a BACKGROUND Systems and instruments for the preventionof electrocution are receiving increasing attention because of a greaterrecent interest in safety generally and because of the expandingopportunities for shock hazards to present themselves. The use ofelectrical appliances and power tools is increasing at a rate fasterthan that of the population. The special shock hazards presented byappliances and power tools, not to mention electrically illuminated andelectrically circulated swimming pools, have stimulated the developmentof ground fault interrupter systems. Ground fault interrupter systemsare intended to sense small differences in current in normally balancedpower lines or cable. These differences may be caused by a leakage ofcurrent from one of the line conductors to ground, thus depriving thereturn line of some of its normal current which would establish abalance or zero difference in current at the sensor. As long as thedifference current is below a predetermined level, typically about 0.005amperes, power should normally be allowed to flow uninterrupted. If alarger difference current occurs, the circuit should be interrupted,since it is then probable that a malfunction of insulation or perhapseven a serious shock to a human being is occurring. I One such groundfault interrupter system is described and claimed in my US. Pat. No.3,614,534 on improvements in Ground Fault Responsive ElectricalProtective Systems" issued Oct. 19, 1971. Ground fault interruptingsystems or ground fault indicators involve challenging technicalproblems, since they should be able to detect very small differences inrelatively large currents. A difference of 5 milliamperes in a normalline current of S amperes is a typical requirement. Although asensitivity of 1 part in 10,000 can readily be achieved in thelaboratory, formidable problems arise in practical field use due tospurious signals which can be confused with real fault currents. Forexample, power line transients due to sudden load changes or lightningcan cause nuisance tripping. Intolerance to frequent nuisance trippingcan cause the users of such equipment to establish sensitivityspecifications at dangerously high levels. Indeed, the ground faultinterrupters accepted for common use in Europe and in Africa are said torespond typically to approximately 25 milliamperes, a level which isconsidered to be well above the shock tolerance of many people. Asteady-state spurious signal frequently encountered is a capacitivecurrent from the high line side to ground. This can be caused by a longburied cable or by discrete capacitors used for radio frequencyinterference filters or the like. Ground fault interrupter systems inthe prior art have no way of discriminating against these harmlessreactive currents which can therefore cause nuisance tripping.

The present invention provides a system for discriminating againstreactive currents, spurious transients, indeed against any electricalsignal, the wave form of which does not correspond to or correlate inphase with the line voltage. The means for this acnuisance tripping canbe significantly reduced.

The question may arise as to whether a reactive fault current mightoccur which could be hazardous but which would fail to actuate theground fault interrupting system embodying this invention. Clearly,however, it is only the real portion of the fault current which cancause dissipation of energy leading to tire hazards in insulation. Theproblem of electrocution is perhaps less obvious, but I have found byexperiment that it is 0 not possible to pass alternating currentsthrough animal tissue or flesh without the production of real"components detectable by the improved ground fault indicating andinterrupting systems described herein.

One startling experiment with a system embodying this invention showsthat .a substantial reactive fault current, simulated by a discretecapacitor connected directly from a power line to ground, does notsignal the interrupter. The inclusion of a turkey carcass in series withthe same capacitor, however, quickly trips the circuit breaker,responding to the existance of a genuine fault. The point to emphasizehere is that the addition of the flesh as a series resistance reducesthe absolute magnitude of the current imbalance, but raises the phasecorrelation sufficiently to actuate the ground fault interrupter system.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a simplified block diagramofa synchronous ground fault interrupting system in accordance with thisinvention;

FIG. 2 is a schematic circuit of a preferred form of the invention; and

FIG. '3 is a schematic circuit of another preferred form of theinvention.

DETAILED DESCRIPTION To illustrate the principles of this invention, thesimplified block diagram of FIG. 1 will serve as a general example of aphase-sensitive ground fault detecting and circuit interrupting systemconstructed according to the principles of this invention. Supplyterminals 10 and 11 are intended to be connected to a source ofalternating current potentials for supplying electrical power overconductors l2 and 13 to any suitable load represented generally at 14.Supply conductors 12 and 13 pass through a differential current sensor15 and circuit breaker 16, although not necessarily in that particularorder. Differential current sensor 15 responds to currents in conductorsl2 and 13 to generate a signal over connections 17 and 18 responsive toany difcomplishmentcan be simple, and the problem of ference in currentsin the supply conductors. Under normal system functioning, the currentsin one conductor are equal and opposite to those in the other conductorand the differential current sensor 15 generates no output signal.

In FIG. 1 two potential trouble points are represented by impedances Zaand Zb respectively. These represent possible paths for current to flowto ground from either side of the load 14. A current in either of thesehypothesized impedances bypasses the normal current path back to thegroundedsupply tering on whether the ground fault is reactive orresistive or both.

Synchronous demodulator 20 connected to receive the ground faultcharacterizing signals over lines 17 and 18 also receives a referencesignal over lines 21 and 22 from the supply conductors 12 and 13. Thesynchronous demodulator 20 has a number of functions, about which morewill be said below. Among these functions is to demodulate the groundfault characterizing signal against the reference signal to generate anin-phase ground fault signal representing the resistive component of theground fault characterizing signal. This in-phase component, small incomparison with the currents in the supply conductors,

should preferably be amplified by amplifier 23 and integrated byintegrator 24. When the integrated signal exceeds a predeterminedthreshold value, the circuit breaker 1'6 interrupts the currents insupply conductors 12 and 13.

Hypothesized impedances Za and Zb can be taken to represent any kind ofleakage path to ground. All leakage paths, however, are not necessarilyfaults. For example either impedance could result from a capacitivecoupling between one of the supply lines and ground; a coupling,furthermore, that dissipates no power and represents no true fault. Suchcurrents pose no shock hazard. In such a case the differential currentsensed by sensor is 90 out of phase with the potentials across thesupply conductors l2 and 13. The synchronous demodulator, sensing anout-of-phase condition, produces no output signal capable of causing thecircuit breaker to open the supply circuit. But should either of thehypothesized impedances contain a resistive component, the results aredifferent. Resistive or in-phase components pose a real safety hazard ifthe current level is sufficiently high- Resistive or in-phasecomponents, because they dissipate power, can cause tires and can bringabout electrocution of individuals. The synchronous demodulator sortsout the in-phase component, and delivers an output signal which, if itis of sufficient magnitude, causes the circuit breaker 16 to trip.

A circuit diagram of one of the preferred embodiments of the inventionis shown in FIG. 2, which contains all of the functional componentspreviously described, identified by the same reference numerals appliedto the block diagram of FIG. 1. In the embodiment the DifferentialCurrent Sensor 15 comprises a differential impedance combinationgenerally of a type described and claimed in U.S. Pat. No. 3,614,534which issued Oct. 19, 1971. The differential impedance combination showncomprises a principal magnetic circuit defined by a toroidal core 30 ofhighly permeable material upon which are wound a plurality of windings31 and 32, one for each of the supply conductors of the power supplycircuit. Windings 31 and 32 are wound such that the common mode currentsflowing within them setup equal and opposite fluxes in the core 30. As aconsequence, the magnetic fluxes due to common mode currents cancel eachother and no potentials are induced in either winding. Each winding doesexperience a potential drop, however, as a result of the IR drop thereinplus a drop due to leakage'reactance times the current. In addition tothe principal magnetic circuit, two auxiliary magnetic circuits areprovided by additional toroidal cores 33 and 34 each linked by arespective one of the windings 31 and 32. About these auxiliary coresmore will be said below. At this point it should be sufficient to notethat the purpose of the auxiliary cores is to improve the sensitivity ofthe differential current sensor to certain types of faults. Each of themagnetic core structures 30, 33 and 34 may be formed of wound Permalloytape. The windings embracing these core structures may have three orfour windings and need not be tightly coupled.

The differential current sensor also comprises summing transformer meansincluding a pair of transformers 35 and 36. The primary winding oftransformer 35 is connected across impedance winding 31, whereas theprimary winding of transformer 36 is connected across impedance winding32. The two secondary windings of transformers 35 and 36 are connectedin series to produce a net differential current signal across conductors1'7 and 18.

The synchronous demodulator 20 is essentially a phase-sensitive detectorincorporating, in this preferred example, four ring-connected diodes 40,41, 42 and 43. The input signal, if any, from the series-connectedsecondary windings of transformers 35 and 36 is impressed acrosscapacitor 44. The output signal is developed across capacitor 45. Areferencesi'gnal to control the switching action of the diode ring is.obtained from line conductors l2 and 13 through a secondary winding oftransformer 46, the primary winding of which is connected across thepower lines. The reference signal so obtained is applied at points ofthe demodulator'which are orthogonal to the input connections, namelybetween point 47 between the secondaries of transformers 35, 36 andpoint 48 at the center of a potentiometer 49 across the outputcapacitor. The potentiometer is adjusted until, with no input signal,the output signal is zero. The R-C circuit 50 included in series withthe reference signal source is includedfor fine tuning purposes toadjust the relative phases between the reference signal source and theinput signal source. This fine tuning may be accomplished atinstallation by temporarily connecting hot line 12 .to ground through asubstantial capacitor and adjusting the values of the R-C tuning circuituntil the output of the demodulator 20 is zero.

After installation is complete, if a fault develops between either oflines 12 or 13 and ground, the imbalance in currents is detected by thedifferential current sensor '15, which delivers asinusoidallyvaryinginput signal across capacitor 44. But the demodulator filters outreactive components of the input signal which are out of phase with theline potential and develops a net direct current output signal acrosscapacitor 45 representing the resistive component of the differentialcurrent in lines 12 and 13. This output signal should preferably beamplified and for this purpose a differential operational amplifier 51is provided.

To provide the d.c. potential to operate the amplifier, the transformer46 may be provided with a second output winding connected to full-waverectifier 52 having filtering capacitors 53 across its output terminals.

Terminals 54a and 55a are connected-to corresponding terminals 54b and55b of amplifier 51. Amplifier 51 may preferably be a differentialamplifier with one of its two input terminals grounded through resistor65. A

difference in potential between its input connections gives rise to anoutput signal, the magnitude of which is determined not only by themagnitude of the input differential, but also by the gain-controllingresistor 56 and the adjustment of the potentiometer 57 across which theoutput signal appears.

The output of operational amplifier 51 is thus a dc. signal having amagnitude proportional to the inphase component of differential currentsin the supply conductors l2 and 13. The output from amplifier section 23is integrated by a low-pass integrating filter comprising resistor 58 inseries with the parallel combination of resistor 59 and capacitor 60.This filter acts as an integrator, the time constant of which, on theorder of seconds, is made as large as possible consistent with thedesired system response time. The signal controlling the response ofcircuit breaker 16 thus appears across integrating capacitor 60.

The circuit breaker 16.istriggered in this illustrative example by asilicon unilateral switch (SUS) 61 connected to ground through the inputwinding of pulse transformer 62. The SUS is a silicon planar, monolithicintegrated electrical circuit having thyristor characteristics closelyapproximately those of an ideal fourlayer diode. It switches atrelatively low voltages from a high resistance condition to a very lowresistance condition. This occurs when the potential on capacitor 60reaches a critical threshold value and causes a resulting pulse to becoupled through transformer 62 to the control electrode of siliconcontrolled rectifier (SCR) 63.

The circuit breaker 16 has its winding 64 connected in series with SCR63 across the line conductors 12 and 13. When SCR 63 is triggered to alow resistance state, the full line potential appears across the circuitbreaker winding 64 causing it to open its contacts, therebydisconnecting the load 14 from the source of potential. As illustrated,the interruption of power also removes operating potential from thecircuit breaker winding. The-circuit breaker should accordingly be of alatching type so that its contacts remain open after actuation untilreset. The advantage of this is to be expected: the system, oncetriggered to disconnect, is not caused to reset by temporaryinterruption and restoration of line potential.

At this point some of the additional advantages from the use of thesynchronous demodulator followed by an integrator used in a ground faultsystem bear emphasis. When two signals of identical frequency areintroduced into a synchronous demodulator, the output is a steady directcurrent with a polarity determined by the relative phases of the twosignals. If the two signals have slightly different frequencies, theoutput signal is no longer a steady direct current, but alternatesinpolarity at a frequency equal to the difference between the inputfrequencies. If the output of the synchronous demodulator is nowintegrated over a period of time much longer than the period of thisdifference frequency, as in the embodiment just described, then theintegrated output signal becomes non-responsive to such input signals.

Indeed, with an integrator-filter having a cutoff frequency ofapproximately 0.6 Hz., it can be shown that the pass-band over which thesystem is fully sensitive is very narrow, extending from about 59.4 Hz.to 60.6 Hz. Beyond this narrow range, signals are attenuated sharply.Consequences of this are that noise and other interference signalshaving frequencies not closely matched to line frequency do not affectthe system.

It is possible for a non-adaptive narrow band-pass filter to duplicatesome of the transmission characteristics of a synchronous detectorfollowed by an integrator. But a ground fault protective system usingsuch a non-adaptive filter would be unworkable, since line frequenciesin ac distribution systems often vary by more than a fraction of acycle. Smaller power distribution plants have frequency tolerances asmuch as :3 Hz. A non-adaptive narrow band filter would be useless inconnection with distribution systems having such variations in supplyfrequency. By contrast, the system here proposed not only discriminatesagainst harmless out-of-phase line current differentials, not onlyrejects transients which might otherwise cause nuisance tripping, butalso neatly and accurately tracks line frequency at all times. With thissystem the passband of the demodulator-integrator combination is alwaysprecisely centered around the actual line frequency.

The insensitivity of the system to spurious signals and reactive currentimbalances in the supply lines results in enhanced sensitivity of thesystem totrue faults. Reference was made in the Background section aboveto an experiment with a turkey carcass. The implications of theexperiment are very significant. With a substantial capacitor connectinghotline 12 to ground differential currents of some magnitude existedbetween the supply conductors, but the system did not react to thesepurely reactive currents. Then an animal carcass was inserted in serieswith the capacitor. The absolute value of the differential currents wasthereby decreased. But because the carcass was a resistive load actuallydrawing power, the inphase portion of the differential currents quicklytripped the circuit breaker, interrupting power to the load and to theturkey car-. cass. The turkey carcass, of course, is a substitute for ahuman body which is similarly resistive.

An example, one of many that could be identified, involves apparatussuch as a high fidelity audio amplifier with capacitors connected fromline to chassis for radio frequency interference filtering. When thechassis is properly grounded, the capacitance currents can causenuisance tripping by conventional ground fault interrupters. Because ofnuisance tripping the sensitivity of the ground fault interrupter may beset undesirably high. But if the chassis is or becomes improperlygrounded through contact with an individuals hand or body, a severeshock can be delivered through the lineto-line chassis capacitor. Aconventional ground fault interrupter with its threshold adjusted highwould not respond. A phase-sensitive ground fault interrupter can have akeener protective sensitivity and still be free of nuisance tripping.

in the description above of the auxiliary magnetic circuits provided bytoroidal cores 33 and 34, it was indicated that more would be said abouthow these auxiliary cores improve the sensitivity of the differentialcurrent sensor to certain types of faults. Ground faults can result frominsulation failure or accidental contact.

to ground from both the high or hot side and the low or neutral side ofa power line. The former presents the more obvious hazard, but groundedneutral short circuits can also cause electrocution of particularlyvul-' nerable victims such as hospital patients wired to electronicinstrumentation. But a more general problem with faults in the neutralline is the automatic desensitization of the ground fault interruptersystem which can thereby make it unresponsive to hot line faults. Thesimultaneous occurrance of ground faults to both sides of the line is areal possibility because in most ground fault interrupter systems aneutral fault can exist undetected indefinitely.

A fault in the neutral line downstream from the differential currentsensor closes a circuit upon the neutral line sensor windings; currentcan circulate between the fault, the intended ground (generally at theutility transformer pole) and windings. These shortcircuited turnsoppose flux changes in the core structure and thus the device isdesensitized.

One solution to the problem of grounded neutral short circuits isdescribed in my U.S. Pat. No. 3,614,534, issued Oct. 19, 1971. Adifferential impedance is arranged to produce a large signal with a lowresistance neutral line fault providing that a substantial load currentis being drawn. The danger remains, however, that high and neutralfaults can occur simultaneouslv with no load.

Desensitization of the differential current sensor by a low resistanceneutral fault can be avoided if some or all of the sensor windings loopauxiliary cores of high permeability magnetic material such as thoseshown at 31 and 32 in FIG; 2. Fluxes induced in the auxiliary coresoppose the circulating currents and the effects of shorted turns areeliminated. These auxiliary cores may become saturated and renderedineffective by substantial load currents but their function is not thenneeded. Two cores are shown in the illustration, but if the installationof the system assures unambiguous identity of the neutral line, only theauxiliary core for that line is needed.

An alternate embodiment of the invention is shown in FIG. 3 whereinthose parts equivalent to corresponding parts of the previousillustration are identified by the same reference numbers. The passivering modulator used in the preceding example is here replaced by amonolithic balanced demodulator 71 principally containing activeelements. This may take the form of an integrated circuit availableunder the model designation MC 1496 from Motorola SemiconductorProducts, Inc., in Phoenix, Arizona. This integrated circuit comprises aquad differential amplifier driven by a differential amplifier with dualcurrent sources, the output collectors being cross-coupled so thatfull-wave balance multiplication of the two input voltagesoccurs. Thatis, the output signal is a constant times the product of the two inputsignals. With both inputs at the same frequency the monolithicdemodulator delivers an output which is a function of the phasedifference between the two signals. Whereas a passive diode ringdemodulator may introduce a signal loss of about 8 db., an activedemodulator such as the model MCl496 may provide a signal gain ofapproximately 10 db. The 18 db. improvement can be used to permit theuse of a smaller differential current sensor or to reduce the gain ofthe post-detection amplifier.

The monolithic demodulator 71 receives its input signals from summingtransformers 35 and 36 and its reference signals from transformer 46.The output signal is delivered from demodulator 71 across a voltagedivider 72 to the input terminals of differential operational amplifier73. This operational amplifier is provided with a source of d.c.operatingpotential at terminals 74 and 75 similar to the way that theoperational'amplifier is shown as receiving its power in FIG. 2. Here,however, the integrator 24 comprises a capaci-v tor 76 in the feedbackloop of the operational amplifier from one of its input ports to itsoutput terminal. This capacitor 76 together with resistor 77 determines.the integrating time constant, typically in the region of from one tofifteen seconds. An additional R-C section comprising series connectedresistor 78 and capacitor 79 is provided, but the principal function ofthese elements is to buffer the operational amplifier from transientsproduced by the triggering of SUS 61.

Furthermore, capacitor 79 provides the pulse energy for use in a powerdistribution system for supplying a.c. I

power to load from at least two supply conductors, one of suchconductors being grounded at the source, comprising: I

differential current sensing means responsive to currents in said supplyconductors for generating signals representing a differential currentcarried by said supply conductors; and

. synchronous demodulator means responsive to said differential currentsignals and to the phase of potentials between said supply conductorsfor deriving a fault-characterizing signal representing the resistivecomponents of said differential .currents in phase with the potentialsacross such supply conductors.

2. A phase-sensitive ground fault protective system of claim 1wherein'said demodulator means comprises principally passive biaseddiodes to derive said faultcharacterizing signals. 3. Thephase-sensitive ground fault protective system of claim 1 wherein saiddemodulator means comprises principally active elements to realize asignal gain in deriving said fault-characterizing signals.

4. The phase-sensitive ground fault protective system of claim 1 furtherincluding means for integrating said fault-characterizing signal toderive an integrated faultcharacterizing signal.

5. The'phase-sensitive ground fault protective system of claim 4 furtherincluding circuit interrupting means for interrupting continuity throughsaid supply conductors when said integrated fault characterizing signalexceeds a predetermined threshold level.

6. A phase-sensitive ground fault protective system for use in a powerdistribution system for supplying ac. power to a load from at least twosupply conductors, one of such conductors being grounded at the source,comprising: V

differential current sensing means responsive to currents in said supplyconductors for generating signals representing a differential currentcarried by said supply conductors; and

phase-sensitive narrow band filter means adaptive to the frequency ofsaid a.c. power for attenuating differential current signals which areof a different frequency then said a.c. power or which are out of phasewith respect to said a.c. power and for integrating differential currentsignals of substantially the same frequency and phase of said a.c. powerto derive an integrated signal representing the time integral of theresistive components of said differential current signals.

7. The phase-sensitive ground fault protective system of claim 6 whereinsaid phase-sensitive narrow band filter means comprises:

a phase-sensitive demodulator responsive to the phase of said a.c. powerand to said differential current signals for generatingfault-characterizing signals; and

an integrator for integrating said fault-characterizing signal to derivesaid integrated signal.

8. The phase-sensitive ground fault protective system of claim 6 furthercomprising circuit interrupting means for interrupting continuitythrough said supply conductors when said integrated signal exceeds apredetermined threshold value.

9. A phase-sensitive ground fault protective system for use in anelectricalcircuit for providing a.c. power to a load from at least twosupply conductors, one of such conductors being grounded at the source,comprising:

signal generating means comprising at least one closed magnetic circuithaving a plurality of electrical windings thereon, each connected inseries circuit with a respective one of said supply conductors and sowound about said magnetic circuit that the fluxes induced in saidmagnetic circuit due to normal currents in said supply conductors are'influx opposition; and

means responsive to a net magnetic flux in said circuit and to the phaseof alternating current potentials between said supply conductors forderiving a fault signal characterizing the resistive component ofunbalanced ground fault currents.

10. The phase-sensitive ground fault protective system of claim 9further comprising circuit interrupting means for interrupting circuitcontinuity between said load and said supply conductors in response tosaid fault signal.

11. The phase-sensitive ground fault protective system of claim 9further comprising integrating means for deriving an integrated faultsignal.

12. The phase-sensitive ground fault protective system of claim 11further comprising means for interrupting circuit continuity betweensaid load and said supply conductors in response to the attainment bysaid integrated fault signal of a predetermined threshold value.

1. A phase-sensitive ground fault protective system for use in a powerdistribution system for supplying a.c. power to load from at least twosupply conductors, one of such conductors being grounded at the source,comprising: differential current sensing means responsive to currents insaid supply conductors for generating signals representing adifferential current carried by said supply conductors; and synchronousdemodulator means responsive to said differential current signals and tothe phase of potentials between said supply conductors for deriving afault-characterizing signal representing the resistive components ofsaid differential currents in phase with the potentials across suchsupply conductors.
 2. A phase-sensitive ground fault protective systemof claim 1 wherein said demodulator means comprises principally passivebiased diodes to derive said fault-characterizing signals.
 3. Thephase-sensitive ground fault protective system of claim 1 wherein saiddemodulator means comprises principally active elements to realize asignal gain in deriving said fault-characterizing signals.
 4. Thephase-sensitive ground fault protective system of claim 1 furtherincluding means for integrating said fault-characterizing signal toderive an integrated fault-characterizing signal.
 5. The phase-sensitiveground fault protective system of claim 4 further including circuitinterrupting means for interruptiNg continuity through said supplyconductors when said integrated fault-characterizing signal exceeds apredetermined threshold level.
 6. A phase-sensitive ground faultprotective system for use in a power distribution system for supplyinga.c. power to a load from at least two supply conductors, one of suchconductors being grounded at the source, comprising: differentialcurrent sensing means responsive to currents in said supply conductorsfor generating signals representing a differential current carried bysaid supply conductors; and phase-sensitive narrow band filter meansadaptive to the frequency of said a.c. power for attenuatingdifferential current signals which are of a different frequency thensaid a.c. power or which are out of phase with respect to said a.c.power and for integrating differential current signals of substantiallythe same frequency and phase of said a.c. power to derive an integratedsignal representing the time integral of the resistive components ofsaid differential current signals.
 7. The phase-sensitive ground faultprotective system of claim 6 wherein said phase-sensitive narrow bandfilter means comprises: a phase-sensitive demodulator responsive to thephase of said a.c. power and to said differential current signals forgenerating fault-characterizing signals; and an integrator forintegrating said fault-characterizing signal to derive said integratedsignal.
 8. The phase-sensitive ground fault protective system of claim 6further comprising circuit interrupting means for interruptingcontinuity through said supply conductors when said integrated signalexceeds a predetermined threshold value.
 9. A phase-sensitive groundfault protective system for use in an electrical circuit for providinga.c. power to a load from at least two supply conductors, one of suchconductors being grounded at the source, comprising: signal generatingmeans comprising at least one closed magnetic circuit having a pluralityof electrical windings thereon, each connected in series circuit with arespective one of said supply conductors and so wound about saidmagnetic circuit that the fluxes induced in said magnetic circuit due tonormal currents in said supply conductors are in flux opposition; andmeans responsive to a net magnetic flux in said circuit and to the phaseof alternating current potentials between said supply conductors forderiving a fault signal characterizing the resistive component ofunbalanced ground fault currents.
 10. The phase-sensitive ground faultprotective system of claim 9 further comprising circuit interruptingmeans for interrupting circuit continuity between said load and saidsupply conductors in response to said fault signal.
 11. Thephase-sensitive ground fault protective system of claim 9 furthercomprising integrating means for deriving an integrated fault signal.12. The phase-sensitive ground fault protective system of claim 11further comprising means for interrupting circuit continuity betweensaid load and said supply conductors in response to the attainment bysaid integrated fault signal of a predetermined threshold value.